All walnut species of the genus Juglans are trees or
large shrubs having shoots with chambered piths, large aromatic compound leaves,
staminate catkins on one year old wood and female flowers on the top of the
current year's twigs. The husked fruit is a false drupe containing a large
woody-shelled nut. All Juglans produce edible nuts, although size and
extrability differ considerably. Most species are highly regarded for their
timber.

The genus Juglans consists of approximately 20 species
grouped taxonomically into four sections: Rhysocaryon, Cardiocaryon, and
Trachycaryon, Dioscaryon. All these species are diploid with 2n = 2x = 32
chromosomes. They can hybridize. The section of Rhysocaryon (black
walnuts) is composed of 16 North and South American Juglans species. The
most important are Juglans nigra L., Juglans hindsii Jeps and
Juglans major Heller used as rootstocks of Persian walnut. The section of
Cardiocaryon consists of species originating from Japan (Juglans
sieboldiana Maxim), China (Juglans catchayensis Dode) and Mandchuria
or the Korean peninsula (Juglans mandshurica Maxim) used as wood
producers and sometimes as rootstocks. Juglans cinerea L. or butternut,
the only species of the Trachycarion section is present in the forests of
north-western Canada and USA. The section Dioscaryon consists solely of
the commercially valuable Persian walnut, Juglans regia L.

Juglans regia is native to the mountain ranges of
Central Asia extending from Xinjiang province of western China, parts of
Kazakhstan, Uzbekistan and southern Kirghizia and from mountains of Nepal,
Tibet, northern India and Pakistan through Afghanistan, Turkmenistan and Iran to
portions of Azerbaijan, Armenia, Georgia and eastern Turkey. In these countries,
there is a great genetic variability in particular ancestral forms with lateral
fruitfulness. During its migration to western Europe, English walnut lost this
character by natural selection on account of competition with other vigourous
forest species such as oaks. They became big trees with terminal fruitfulness. A
small remnant population of these J. regia trees have survived the last
glacial period in Southern Europe but the bulk of the wild J.regia germplasm in the Balkan peninsula and much of Turkey was most
likely introduced from eastern Turkey by commerce and settlement several
thousand years ago. Four centuries BC Alexander The Great introduced in
Macedonia ancestral forms with lateral fruitfulness from Iran and Central Asia.
They hybridized with terminal bearing forms to give lateral bearing trees. These
lateral bearers were spread in Southern Europe and Northern Africa by Romans.
Recent prospections in walnut populations of the Mediterrean Basin allowed to
select interesting trees of this type. In the Middle Ages the lateral bearing
character was introduced again in southern Turkey by merchants travelling along
the Silk Road. J. regia germplasm in China is thought to have been
introduced from Central Asia about 2 000 years ago and in some areas has become
naturalized. Cultivated distribution now includes North and South America
(Chile, Argentine), Australia, New Zealand, South Africa and Japan. So Persian
walnut is grown from 30 to 50 degrees of latitude in the Northern hemisphere and
from 30 to 40 degrees in the Southern hemisphere.

Though Juglans regia L. is self compatible, its
dichogamy encourages allogamy. Thus, the genetic structure of the Persian walnut
is fairly heterozygous. Cultivated varieties generally well adapted to climatic
conditions of the different production areas, often lack some important
agronomic characteristics. It is therefore useful to select in natural
populations or create through hybridization new cultivars combining characters
of improved climate adaptation (late budbreak, low chilling requirement or
winter hardiness), early fruiting and high productivity (lateral fruitfulness),
disease tolerance (blight and anthracnose), hypersusceptibility to Cherry leaf
roll virus, high fruit and kernel quality. This is possible given the very large
and so far unexploited variability within the Juglans regia L. species.
Hybridization programs to create new walnut varieties are relatively less
developed than those of the other fruit species. The main programs are carried
out at UC Davis (USA) and at the Fruit and Vine Research Station in Bordeaux
(France). On the other hand, during the last decade, many prospections were
carried out in natural populations in the main producer countries in particular
in Greece, Iran, Italy, Morocco, Portugal, Romania, Spain, Turkey, and
ex-Yugoslavia (Yugoslavia was left as country of origin in all tables in cases
where no further geographical indication was available to identify the current
country). New interesting genitors or cultivars are beginning to be released
from these breeding and selection works.